In general, a bill processing apparatus, which handles a bill as one of the embodiments of the paper sheet, is incorporated into a service device such as a game medium rental machine installed in a game hall, a vending machine or a ticket-vending machine installed in a public space, or the like which judges the authenticity of the bill inserted from a bill insertion slot by a user and provides various types of products and services in accordance with a value of the bill having been judged as authentic.
Generally, in a bill authenticity judgment process, for example as disclosed in Patent Reference 1, a bill moving along the bill travelling route is irradiated with light, and the transmitted light and reflected light from the bill are detected by a light receiving sensor and compared with reference data stored in dictionary data. More specifically, the transmitted light data and reflected light data from the conveyed bill are acquired, converted for example to brightness information, and compared with normal data.    [Patent Reference 1] Japanese unexamined patent application publication No. H6-60242